Cytotoxic T lymphocytes (CTL) are key immune effector cells in controlling viral infections and tumors. However, their differentiation in vivo is only partially understood. We have found that a novel APC-CD8 T cell interaction through an a(1,3) fucosylated P-selectin glycoprotein-1 (PSGL-1) is required to generate effector CTL. In its absence, Vaccinia virus-infected mice deficient in a(1,3) fucosyl transferase IV+VII (FT-/-) are deficient in generating effector CTL, although they generate activated CD8 T cells capable of viral-specific proliferation. In Aim 1, we will breed FT-/- mice to mice transgenic (Tg) to lymphocytic choriomeningitis (LCMV) glycoprotein TCR. This system will enable us to study the role of FT in antigen-specific CTL response after LCMV challenge. We will determine if FT is required on APC or on CD8 T cells in adoptive transfer experiments. Leukocyte adhesion deficiency type II (LADII), a human disease due to defective fucose metabolism allows us to study the role of fucosylated PSGL-1 in humans. We have also produced transgenic mice whose T cells express the jellyfish Victoria acquoria-derived green fluorescent protein (GFP-Tg). When these mice are immunized, the effector CTL lose GFP expression, whereas naive and activated pre-effector T cells continue to express GFP. Thus, GFP-Tg mice provide a powerful tool to separate differentiated effector from pre-effector CTL. Using this system, we have identified several previously unrecognized molecular changes that occur during effector differentiation. Thus, to study the molecular mechanisms that are defective in the absence of FT, in aim 2 we will breed FT-/- mice to GFP-Tg mice and analyze their antigen activated CD8 T cells for molecular differences from FT+/+ CD8 T cells. Since selectin-ligand deficient FT-/- mice, but not selectin-deficient mice are defective for CTL generation, a non-selectin receptor may exist for the fucosylated PSGL-1. Indeed, we have identified such a counter-receptor on activated T cells using soluble recombinant PSGL-1 tetramers. Thus in Aim 3, we will isolate, clone and characterize the molecule.